In recent years, so-called polymer composite materials (nanocomposites), which are respectively nanoscale composites of metal oxide particles with polymers, have been under research and development because polymer composite materials may be able to express unconventional and characteristic physical properties. In particular, a polymer composite material including metal oxide particles having a particle diameter ranging from 1 nm to 100 nm can exhibit unconventional physical properties. Polymer composite materials are thus expected to be used in various applications, e.g., for an optical material, an electronic material, a coating material, a gas barrier material, and a photocatalyst.
With regard to the application for, e.g., an optical material, research has been conducted on how to uniformly disperse highly refractive metal oxide particles in a transparent polymer, the metal oxide particles having a primary particle diameter sufficiently smaller than a wavelength of visible light, because uniform dispersion of such metal oxide particles will not impair visible light transmissivity. The above dispersion, if achieved, can bring about production of a polymer composite material which not only has a high transparency and a high moldability, but also has a high refractive index unachievable with a resin solely.
As one of the above researches it has been studied to produce a transparent and highly refractive polymer composite material by including, as the above metal oxide particles, highly refractive metal oxide particles such as titanium oxide particles and zirconium oxide particles so as to form a composite of such metal oxide particles with a thermosetting resin or a photo-curable resin.
However, a polymer composite material including a thermosetting resin or a photo-curable resin as a polymer matrix is extremely limited in terms of, e.g., a composite forming method and a molding method.
In view of this, research has been conducted on a polymer composite material including a thermoplastic resin as a polymer matrix, which is a medium in which metal oxide particles are to be dispersed. The use of a thermoplastic resin makes it possible to select one from among a variety of generally used molding methods such as injection molding, as appropriate according to intended use of the polymer composite material. Metal oxide particles can be dispersed in a thermoplastic resin by, e.g., a method involving use of a dispersing agent or a surface treatment agent.
One specific example of the polymer composite material including a thermoplastic resin as a polymer matrix is a polymer composite material including metal oxide particles dispersed in a polyimide (see, for example, Patent Literature 1).
However, the polymer composite material disclosed in Patent Literature 1 includes a polyimide having a high melting temperature and metal oxide particles. The polymer composite material hence has a high melting temperature. This makes it difficult to mold this polymer composite material by a generally used molding method such as injection molding.
Another known polymer composite material includes a polymer and ultrafine metal oxide particles each having a particle diameter ranging from 1 nm to 100 nm and having a surface modified with either an acidic group or a combination of an acidic group and a basic group (see, for example, Patent Literature 2). According to Patent Literature 2, the metal oxide particles can be dispersed in a thermoplastic polymer containing an electron-donating group. Specifically, Patent Literature 2 teaches the following technique: A thermoplastic resin containing an electron-donating group is used a polymer matrix. Metal oxide particles to be used each have a surface modified with either an acidic group or a combination of an acidic group and a basic group by use of an organic modifier. The metal oxide particles are dispersed in the polymer matrix by means of electron transfer to trap sites on the surfaces of the metal oxide particles, thus producing a nanocomposite.
Another known method for producing a transparent and highly refractive polymer composite material is a method which improves compatibility between metal oxide particles and a polymer. A known example of the method is a method in which a functional group capable of covalently bonding with both metal oxide particles and a polymer is introduced so that the metal oxide particles are bonded with the polymer (see, for example, Patent Literature 3).
Citation List
Patent Literature 1
Japanese Patent Application Publication, Tokukai, No. 2001-348477 A (Publication Date: Dec. 18, 2001)
Patent Literature 2
Japanese Patent Application Publication, Tokukai, No. 2003-73558 A (Publication Date: Mar. 12, 2003)
Patent Literature 3
Japanese Patent Application Publication, Tokukai, No. 2002-47425 A (Publication Date: Feb. 12, 2002)
Patent Literature 4
Japanese Patent Application Publication, Tokukai, No. 2003-261329 A (Publication Date: Sep. 16, 2003)
Non Patent Literature 1
Yukiya Hakuta, Haruo Ura, Hiromichi Hayashi, Kunio Arai, “Effect of water density on polymorph of BaTiO3 nanoparticles synthesized under sub and supercritical water conditions”, Materials Letters 59, p. 1389-1390 (2005).